EP0612126B1 - Method for stable operation of an active-mode-locking optical-fiber laser generator - Google Patents

Method for stable operation of an active-mode-locking optical-fiber laser generator Download PDF

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Publication number
EP0612126B1
EP0612126B1 EP94101253A EP94101253A EP0612126B1 EP 0612126 B1 EP0612126 B1 EP 0612126B1 EP 94101253 A EP94101253 A EP 94101253A EP 94101253 A EP94101253 A EP 94101253A EP 0612126 B1 EP0612126 B1 EP 0612126B1
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Prior art keywords
active mode
locked laser
laser according
stably operating
filter
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Expired - Lifetime
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EP94101253A
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German (de)
English (en)
French (fr)
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EP0612126A1 (en
Inventor
Flavio Fontana
Sergio Bosso
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Pirelli and C SpA
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Pirelli Cavi e Sistemi SpA
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06791—Fibre ring lasers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08—Construction or shape of optical resonators or components thereof
    • H01S3/08018—Mode suppression
    • H01S3/08022—Longitudinal modes
    • H01S3/08027—Longitudinal modes by a filter, e.g. a Fabry-Perot filter is used for wavelength setting
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
    • H01S3/1068—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using an acousto-optical device
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
    • H01S3/107—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using electro-optic devices, e.g. exhibiting Pockels or Kerr effect
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
    • H01S3/1106—Mode locking
    • H01S3/1109—Active mode locking

Definitions

  • Pulses of the above kind having a duration 6t ⁇ 100 ps commonly referred to as very short pulses or “solitons” (under guided propagation conditions), apply in particular to high speed digital telecommunications (up to 10 GBit/s) and in addition to the accomplishment of optical instruments, tests on semiconductor components and also to remote measurements, in topography or in the atmospheric radar field for example.
  • mode-locking pulse laser generators are known; by mode-locking it is meant a process by which, in a laser, several modes oscillate in a mutual constant-phase relationship, by effect of a non-linear-behaviour element present in the laser cavity.
  • Mode-locking laser generators of the passive type comprise optical-fibre laser devices using erbium doped fibres (the laser shown in Fig. 8, for example) in which a fibre ring containing an optical amplifier, acts as a saturable absorber, for example to the wavelength of 1.5 ⁇ m; pulses generated in these lasers depend on the laser ring size.
  • Optical-fibre laser devices are also known in which an active modulation device of the electro-optical type is inserted in an optical path forming a laser cavity so as to force the laser to generate mode-locking pulses at the desired frequency.
  • Such devices are referred to as active mode-locking devices because the modulation device present therein operated from the outside acts on the modes generated in the laser, enabling amplification up to a level keeping the laser emission to the only modes selected by the applied modulation.
  • Passive mode-locking generators the operation of which leaves out of consideration an exciting modulation, have the particular feature that they have a soliton-pulse emitting frequency closely depending on the physical features of the generator, in particular the sizes of the fibre ring forming the laser system, in which for each reflection a single pulse travels.
  • the pulse emitting frequency depends on the exciting frequency of a modulator and it enables a great number of locked pulses simultaneously travelling in the ring to be present and therefore enables the pulse emitting frequency to be previously selected and to be much higher than in the passive-type devices, on the order of GHz for example.
  • mode-locking laser generators can be provided with a filter adapted to enable selection of the emission wavelength of the emitted pulses within a given frequency band, in the 1530-1560 nm band for example, which is the band commonly used in telecommunications, both for feeding different sources on the same fibre and for adaptation to the chromatic dispersion features of the fibre in the connected line.
  • a ring laser of the above type comprising an erbium-doped active fibre, a pump laser feeding the optical fibre, an optoisolator, an electrooptical width modulator made of LiNbO 3 , in which a Fabry-Perot tuning filter has been used for selection of the emission wavelength.
  • the critical behaviour is deemed to be caused by alterations due both to temperature changes and vibrations or the like in the apparatus.
  • a fibre laser using an active erbium-doped optical fibre of the active mode-locking type capable of wavelength tuning, can have a high operation stability and resistance to mechanical perturbations if it is accomplished in the absence of spatial-movement adjustment devices.
  • an active mode-locking laser system in which a filter for tuning the emission wavelength is made in planar optics, enables a high emission stability to be reached in operation even in the absence of specific backing and mechanical-stabilisation structures.
  • said fluorescent dopant in the active fibre is erbium.
  • the means for feeding pumping energy comprises a dichroic coupler connected to one end of the active fibre and to a pumping laser of predetermined wavelength.
  • the electrooptical drivable modulator is a Mach-Zehnder intensity modulator having an optical channel-shaped waveguide, associated with an electronic preselectable-frequency generator.
  • the drivable electroptical modulator is a phase modulator having an optical channel-shaped waveguide associated with an electronic preselectable-frequency generator.
  • the filter for selection of the wavelength is an acoustooptical filter and, more particularly, the acoustooptical filter is a filter having an optical channel-shaped waveguide made of lithium niobate with a lithium niobate crystal substrate.
  • the modulator and filter are formed on a single common crystal substrate.
  • the common substrate for the modulator and filter consists of a lithium niobate crystal comprising at least one optical channel-shaped waveguide.
  • the modulator and filter are formed with a single common crystal substrate and the drivable modulator is a Mach-Zehnder intensity modulator; alternatively, the drivable modulator is a phase modulator.
  • the optical-waveguide filter substrate is a crystal made of lithium niobate having a x-cut orientation and y propagation.
  • the common substrate for the acoustooptical filter and electrooptical modulator is a crystal made of lithium niobate having a x-cut orientation and y propagation.
  • the optical-waveguide monolithic filter comprises a temperature control and adjustment means rigidly connected to the crystal substrate; in particular, the temperature control and adjustment means comprises a Peltier cell.
  • the acoustooptical filter both in an independent form and in combination with the electrooptical modulator, comprises an acoustic waveguide which is parallel to and overlaps the optical waveguide, of predetermined length, at one end of which a one way-transducer is present which is adapted to generate an acoustic wave in the presence of means for polarizing an optical signal guided within the optical waveguide.
  • the laser cavity is in the form of a ring and preferably comprises at least one optoisolator.
  • the laser cavity is of linear configuration and comprises optical reflection means at the respective ends thereof.
  • An optical-fibre active mode-locking laser is generally made according to the diagram shown in Fig. 1.
  • Such a laser comprises an active fibre 1 doped with a fluorescent substance, erbium for example, connected at one end thereof to the trailing fibre 2 of a dichroic coupler 3 having one leading fibre 4 connected to a pump laser 5 and a second leading fibre 6 connected to an optical-fibre ring 7, in which an optoisolator 8 and a directional coupler 9 through the trailing fibre 10 of which the generated signal is emitted, are inserted; a polarization controller 11 enables the polarization characteristics of the different fibres forming the ring or, more generally, the laser cavity to be compensated for.
  • a polarization controller 11 enables the polarization characteristics of the different fibres forming the ring or, more generally, the laser cavity to be compensated for.
  • the laser thus made acts through the feeding of luminous pump energy to the active fibre, at a wavelength adapted to excite to a laser emission state the fluorescent dopant contained in the active fibre; from such laser emission state said dopant can decay to a base state, with emission of a light signal to a predetermined wavelength both spontaneously and following passage through the fibre of a light signal of the same wavelength.
  • Such a light signal will travel through the active fibre a great number of times, by virtue of the laser cavity configuration and consequently it will be multiply amplified to a level sufficient to overcome losses, thereby generating a laser signal or emission signal that can be drawn out.
  • An electrooptical amplitude or phase modulator 12 is in addition inserted in the fibre ring and enables operation of the active mode-locking laser to be carried out by virtue of an outer electric driving of appropriate frequency through a radio-frequency square or sine wave generator, not shown.
  • radio-frequency it is intended a frequency of 100 MHz to 2-3 GHz or even higher.
  • the laser may be tuned to the emission wavelength through a filter 13.
  • Such a filter in the known art typically consists of a Fabry-Perot filter, substantially comprising a pair of variable-index lenses connected to the ends of two optical fibres having two respective parallel and facing faces provided with reflecting coatings; the distance separating the two faces between which the signal is submitted to a variety of reflections generating a constructional interference at a wavelength depending on the distance itself, can be adjusted through drivable means such as piezoelectric elements, thereby enabling selection of the desired wavelength.
  • a laser of the above type made in a laboratory with the use of a Fabry-Perot filter has required adjustments of the driving frequency of the electrooptical modulator every 15 minutes for maintaining the mode-locking operation, 3-4 KHz variations in the frequency modulation being provided.
  • a mode-locking laser in accordance with the invention adapted to offer the required reliability for its practical use has been made with the configuration shown in Fig. 2 in which the components already described in the general structure seen in Fig. 1 are identified by the same reference numerals.
  • the laser comprises an erbium-doped active fibre 1, connected to a dichroic coupler 3 fed from a pump laser 5 acting at a 980 nm wavelength; the end of the active fibre 1 opposite the dichroic coupler 3 is connected, through an optoisolator 8 enabling one-way propagation in the ring, to the directional coupler 9 from the trailing fibre 10 of which the emitted signal is drawn; provision is also made for a polarization control element 11.
  • Mode-locking is achieved through an amplitude modulator 14, of the type providing for a Mach-Zehnder interferometer connected with an electronic radio-frequency (rf) pulse generator 15.
  • rf radio-frequency
  • the emission wavelength is adjusted through an acoustooptical filter 16 in turn connected to a rf generator 17.
  • the optical fibre 10 issuing from the directional coupler 9 has been connected to a receiving apparatus R through which the laser emitted signals have been analyzed.
  • the overall length of the ring was about 20 m.
  • the dichroic coupler 4 is a fused-fibre coupler formed with two mono-mode fibres of 980 and 1536 nm wavelength, the variation of the optical output power of which depending on polarization is ⁇ 0.2 dB.
  • Dichroic couplers of the stated type are known and produced for example by Gould Inc., Fibre Optic Division, Baymeadow Drive, Glem Burnie, M.D. (USA) and Sifam Ltd., Fibre Optic Division, Woodland Road Torquay Devon, (GB).
  • Lasers of the above type are for example produced by: David Sarnoff Research Center, Washington Rd, Princetown, N.J. (USA).
  • the directional coupler 9 is a device adapted to divide an optical signal present at one of its inlets into predetermined proportions at the outlets thereof.
  • the directional coupler 9 is a coupler of a commercial type having a 50/50 division ratio, for example a model 1550 POH 50/50 2x2 produced by the above mentioned firm Gould Inc.
  • the polarization control element 11 consists of several optical fibre coils disposed in succession and supported in an adjustable manner relative to a common alignement axis for carrying out the desired adjustment.
  • the optoisolator 8 is an optical isolator providing for polarization control of a type independent of the polarization of the transmission signal, in which isolation is greater than 35 dB and reflectivity lower than -50 dB.
  • the isolator used is a model MDL I-15 PIPT-A S/N 1016 available from ISOWAVE, 64 Harding Avenue, Dover, New Jersey, USA.
  • the receiving apparatus R consists of a non co-linear autocorrelator, model FR-1 3KR produced by FEMTO-CHROME INC., USA.
  • the emission band has been also measured through an optical spectrum analyzer; the apparatus used is model TQ8346 manufactured by AVANTEST, 300 Knightsbridge Parkway, Lincolnshire IL 60069 Illinois, USA.
  • the intensity modulator is a Mach Zehnder interferometer modulator having a channel-shaped waveguide made of lithium niobate (LiNbO 3 ) and a traveling wave electrode of a commercially available type; the model used is produced by the applicant and put on sale under the brand name PIRMZM 15 3 00.
  • the acoustooptical filter 16 is shown in detail in Figs. 3 and 4.
  • the filter 16 is formed from a substrate 18 consisting of a LiNbO 3 x-cut crystal on which an optical channel-shaped waveguide 19 having a y-propagation orientation and being mono-mode in the emission wavelength band, has been made by diffusion of titanium, thanks to which the refraction index of the subtrate lithium niobate is locally increased so as to confine the optical signal within the required path.
  • a pair of acoustooptical transducers 20 is present, which consist of metal plates having several comb-shaped electrodes alternated with each other and overlapping the waveguide, adapted to generate, by piezoelectric effect, a sound wave propagated in the crystal, as a result of an electrical excitation applied thereto.
  • An acoustic waveguide 22 is defined on the sides of the optical waveguide 19; it is bounded by two side areas 23 in which titanium is diffused in the lithium niobate substrate, in order to increase the propagation speed of a sound wave in such side areas relative to the central area 22, so that the sound wave is guided into said central area.
  • a polarizer 24 of the TE feed through type is present upstream of the electrodes 21, whereas a polarizer 25 of the TM feedthrough type is present downstream of the electrodes 21.
  • the TE feed through polarizer 24 is composed of a dielectric material (such as silicon oxide) layer of predetermined thickness applied to the optical waveguide 19 on which a metal layer is overlaid; this polarizer enables the polarized light signal component to pass in the plane of the LiNbO 3 crystal surface.
  • a dielectric material such as silicon oxide
  • the TM feed through polarizer 25 is in turn composed of two protonic exchange areas made sideways of the waveguide over some millimeters in length; this polarizer enables the polarized light signal component to pass in a plane orthogonal to the LiNbO 3 crystal surface.
  • a sound absorber 26 is present at a predetermined distance from the TM feedthrough polarizer 25, on the opposite side with respect to the electrodes 21; it consists of a sound absorbent material layer applied to the crystal surface, adapted to absorb the acoustic wave and therefore prevent it from further interaction with the light wave.
  • interconnecting fibres 27 As diagrammatically shown in Fig. 4, connected to the ends of the waveguide 19 are interconnecting fibres 27 by which the filter is connected to the remaining laser components.
  • the rf generator 15 driving the intensity modulator had a selectable frequency ranging between 100 mHz and 2.4 GHz, in relation to the desired emission frequency of the optical pulses generated by the laser (equal to the frequency of generator 15) and the characteristics of the laser cavity which possesses a fundamental (transit) frequency of its own of which the excitation frequency must be a multiple; the frequency used in the experiment described was approximately 635 MHz.
  • the rf generator 17 driving the acoustooptical filter had a selectable frequency ranging between 170 MHz and 180 MHz, for selecting the wavelength of the optical laser pulses in the desired field (1530 ⁇ ⁇ ⁇ 1560).
  • the intensity modulator is employed at frequencies higher than 300 MHz, both in order to have pulses provided with the best features particularly in terms of pulse shortness and constancy, and in view of the desired applications, in particular for high speed telecommunications.
  • the acoustooptical filter was provided with a heat control and adjustment device 28, which has not been turned on during the experiment and the whole laser assembly has been tested at room temperature.
  • the laser having the described structure has been set in operation in a laboratory and has shown a steady operation, emitting pulses at regular intervals over all the wavelengths in the selected wavelength field (between 1530 and 1560 nm), an adjustment of the emission frequency of the rf generator 15 lower than 1 KHz being required after eight hours of continuous operation, for the purpose of optimizing the profile of the emitted pulses only; the tested emission wavelengths have been steady within 0.1 nm-corresponding to the resolution of the spectrum analyzer.
  • the temperature in the environment in which the laser has been tested is deemed to have oscillated in the range of ⁇ 2°C relative to an average value of 21° in the progress of the test, while at the same time no particular measures have been adopted for eliminating or restraining the mechanical disturbances.
  • Table 1 reproduces, by way of example only, some of the selected emission wavelengths and the relevant driving frequencies of the electrooptical modulator and acoustooptical filter.
  • Mode-locking frequency MHz
  • Filter frequency MHz
  • Emission wavelength 635.693 177.092 1532.8 635.691 176.492 1538.5 635.689 175.492 1546.5 635.688 174.792 1552.0 635.686 173.992 1558.3
  • thermoregulation device 28 applied to the acoustooptical filter comprising a Peltier cell, a heat sensor and the relevant regulation circuit, by which the filter temperature can be maintained constant in a very precise manner even in the presence of important environmental temperature changes.
  • the monolithic structure and the reduced sizes of the filter the overall length of which does not exceed a few centimetres and the thickness and width of which are in the order of a few millimeters, enable its temperature to be easily controlled both by a heat insulator and by the thermoregulation device.
  • the polarization control element 11 has been utilized in the laser of the described example in order to compensate for the signal polarization variations caused by the fibres and components used for producing the laser; however if polarization-maintaining fibres and components are used in the laser the polarization element 11 can be eliminated.
  • an acoustooptical integrated-optics filter enables the filter and modulator, in a further embodiment capable of offering further advantages, to be made of a single planar-optics element 29, as shown in Figs. 7, 8, 9.
  • a portion 31 forming the acoustooptical filter the components of which are identified by the same reference numerals used in Fig. 3 and to the description of which you may refer and, consecutive thereto, a portion 32 forming an electrooptical modulator having a Mach-Zehnder interferometer, thereby constituting an integrated modulation and tuning assembly.
  • the optical waveguide 33 forming the extension of the waveguide 19 of filter 31 is divided into two portions 33a and 33b which are subsequently joined together at the output end 34 of the modulator.
  • the electrodes 35, 36, 37, alternately connected to the outputs of the electronic rf generator 15 generate, by an electrooptical effect, a phase displacement between the two portions into which the signal has been divided, so that, on joining, the desired intensity modulation is caused by interference.
  • the integrated embodiment described enables the laser structure to be greatly simplified, in particular by virtue of the fact that it enables the number of connections necessary between the fibres and various components to be reduced.
  • the electrooptical modulator integrated in a single substrate in the assembly 29 may consist of a phase modulator 38; this frequency modulator is formed with an optical waveguide 39 made in the extension of the waveguide 19 in the substrate 30 crystal, and alongside it there are the electrodes 40, 41 connected to the rf generator 15.
  • the electric signals applied to the electrodes 40, 41 modify the light propagation features within the optical waveguide 39 which result in a phase modulation of the optical signal that propagates in the waveguide itself.
  • This modulation introduces a linearly increasing phase displacement between the modes generated in the laser and causes the locking thereof, thereby giving rise to the desired very short pulses.
  • the acoustooptical filter may preferentially comprise a second TE feedthrough polarizer 24, shown into brackets in Figs. 3, 8, 9, adapted to enable passage of the desired polarization state alone, eliminating possible residual components; in another alternative embodiment, should the fibres forming the laser cavity be capable of efficiently maintaining the polarization state, the TE feedthrough polarizer could be omitted.
  • the optoisolator 8 and polarization selector 11, if any, can be disposed within the laser cavity; the pump laser 5 too and the relevant dichroic coupler 3 can be disposed in connection with either end of the active fibre 1, provided that the pumping energy can be supplied to the active fibre itself without important attenuations.
  • an active fibre 1 is inserted in a laser cavity bounded by a high-reflectivity mirror 42 and a part reflecting output coupler 43; the pump laser 5 feeds the active fibre 1 through the coupler 3 and the integrated modulation and tuning assembly 29, together with the relevant rf generators 15, 17, is put into the laser cavity defined by the reflecting elements 42 and 43.
  • the assembly 29 in analogy with the above description, may be replaced by an electrooptical modulator and an acoustooptical filter separated from each other.
  • tunable filters having different configuration than the one described fall within the scope of the present invention, as well as possible different modulator configurations, provided that the modulator is adapted to generate active mode-locking pulses devoid of elements subjected to spatial movements which are susceptible of undergoing or causing accidental deformations of their own spatial configuration and the size of the laser cavity in which they are fitted.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Optics & Photonics (AREA)
  • Lasers (AREA)
  • Glass Compositions (AREA)
EP94101253A 1993-02-19 1994-01-28 Method for stable operation of an active-mode-locking optical-fiber laser generator Expired - Lifetime EP0612126B1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
ITMI930325A IT1263613B (it) 1993-02-19 1993-02-19 Generatore laser a fibra ottica a concatenamento modale attivo
ITMI930325 1993-02-19

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EP0612126A1 EP0612126A1 (en) 1994-08-24
EP0612126B1 true EP0612126B1 (en) 1999-04-07

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US (1) US5381426A (ja)
EP (1) EP0612126B1 (ja)
JP (1) JPH06302881A (ja)
CA (1) CA2114089C (ja)
DE (1) DE69417611T2 (ja)
ES (1) ES2131597T3 (ja)
IT (1) IT1263613B (ja)

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ITMI930325A0 (it) 1993-02-19
CA2114089C (en) 1999-05-11
US5381426A (en) 1995-01-10
IT1263613B (it) 1996-08-27
ITMI930325A1 (it) 1994-08-19
CA2114089A1 (en) 1994-08-20
EP0612126A1 (en) 1994-08-24
DE69417611T2 (de) 1999-10-28
JPH06302881A (ja) 1994-10-28
DE69417611D1 (de) 1999-05-12
ES2131597T3 (es) 1999-08-01

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